In recent years, clinical imaging diagnosis has been progressing remarkably, and the importance of diagnosis and therapy methods based on the use of magnetic resonance imaging (hereinafter referred to as MRI), radiography, ultrasonography, X-ray CT (computed tomography), scintigraphy, etc. has been increasing. Especially, the MRI enables sectional images taken in many arbitrary directions to be obtained with excellent tissue resolution while avoiding dose with radiations. In addition, the MRI provides functional information such as bloodstream, diffusion and temperature. Therefore, the usefulness of the MRI is highly evaluated in the imaging diagnosis field.
Attendant on the recent progress of the MRI-related hardware and imaging sequence, not only the imaging diagnosis but also clinical application of interventional MRI, in which diagnosis and therapeutic procedures are conducted under MRI observation, have been paid attention to. Researches of application of these procedures are under way, the application not being limited to non-vascular areas such as percutaneous puncture biopsy, various drainages, tumor cauterization, etc. but including vascular areas such as angioplasty, stent implanting technique, inferior aorta filter implanting technique, etc.
At present, the procedures relating to diagnosis and therapy in the vascular areas are conducted mainly under X-ray fluoroscopic observation. While only two-dimensional images can be obtained in X-ray fluoroscopy, three-dimensional images can be easily obtained in MRI. The three-dimensional images form information which is very useful in understanding complicated vascular anomalies, such as cerebral arteriovenous malformation and aneurysm. In addition, the various kinds of functional information that can be obtained by MRI cannot be obtained by X-ray fluoroscopy. Besides, the time taken for each of the diagnosis and therapeutic procedures in the vascular areas is about 3 hours on average, and may be 8 hours or longer in difficult cases. During the time required, the patient and the person involved in the medical work are continuously exposed to harmful radiations. In addition, the iodine-based contrast agent used in X-ray fluoroscopy is higher in harmfulness to the human body, as compared with the contrast agents used in MRI. Thus, conducting the diagnosis and therapy in the vascular areas under MRI observation has many merits, as compared with that under X-ray fluoroscopic observation.
However, most of the medical apparatuses used in the diagnosis and therapy in the vascular areas are formed of polymeric material, metal, ceramic or composite material thereof, which makes it impossible to obtain appropriate signals thereof in MRI and which is not visible under MRI. For safe and accurate procedures, therefore, it is desired to establish a technology for visualizing the medical apparatus under MRI.
As a technology for visualizing the medical apparatus under MRI, there have been proposed two types, i.e., an active tracking and a passive tracking. In the active tracking, one or more radio-frequency (RF) coils is incorporated into a medical apparatus such as a catheter, position is computed by a computer based on a magnetic resonance signal detected by the coil, and the computed result is displayed in the state of being superposed on a previously obtained anatomical image. According to this tracking, however, only the position where the coil is attached is visible, so that it is impossible to grasp the total image of the apparatus. Therefore, there is a limit to visualize a flexible apparatus such as catheter. Although incorporation of a multiple coil may be contemplated, it is unfavorable because it influences the mechanical properties of the catheter, probably spoiling the intrinsic functions of the catheter. Besides, heating of the apparatus due to an RF-induced current would also produce a problem.
On the other hand, in the passive tracking, a medical apparatus is visible based on a loss in a magnetic resonance signal. As an example of the passive tracking, there is a technology in which a nonmagnetic material not having a detectable magnetic resonance signal, such as plastic, is used to form the apparatus, and the apparatus is depicted as a no-signal area in an MRI image. This method is superior to the active tracking in that the entire image of the apparatus can be grasped, but the method has a problem in that the loss of signal relevant to the apparatus may be confused with the loss of signal due to air, flowing blood or the like. As another example of the passive system, there is a technology in which a material having a magnetic susceptibility different from those of the peripheral tissues is used to form the apparatus, and the apparatus is depicted by utilizing the distortion of image (artifact) of the periphery of the apparatus due to a magnetic susceptibility effect. However, there is a problem in that the magnetic susceptibility effect is dependent on the orientation of the apparatus relative to the static magnetic field in MRI, and the dimensions of the apparatus cannot be accurately visualized in the image obtained.
As a means for solving this problem, there has been devised a technology of visualizing a medical apparatus into a high signal in a magnetic resonance (hereinafter referred to as MR) image by utilizing the shortening effect on the relaxation time of proton arising from MR of a paramagnetic ion chelate complex. For example, there have been disclosed a method of chemically fixing a paramagnetic ion chelate complex onto a surface of a substrate (base material) constituting a medical apparatus (Patent Document 1), and a method of forming a coating on a surface of a medical apparatus (Patent Documents 3 and 4) by using a water-swellable polymer containing a paramagnetic ion chelate complex (Patent Document 2). By these systems, the whole part of a flexible apparatus such as catheter can be visualized in accurate dimensions.
Meanwhile, medical apparatuses for use in diagnosis and therapy in the vascular areas have to be provided with surface lubricity for reducing damage to the tissues and for enabling assured access of the apparatus to a target location. For this purpose, a method has been disclosed in which the coating film provided for generating a magnetic resistance signal is further coated with a film having surface lubricity (Patent Document 4, and Non-patent Document 1). However, this approach has problems in that the presence of multiple layers of coatings leads to much time required for development of visibility (in the case of Non-patent Document 1, visibility is developed after 15 hours from the moment of immersion in water), that the large coating thickness leads to high possibility of peeling, and that a multiple coating process are required.
As above-mentioned, interventional MRI is of great use in the vascular areas and, therefore, is expected to be advanced remarkably in the future. According to the prior art, however, no preferable method has been obtained that ensures that, when applied to diagnosis and therapy under MRI, the whole part of a medical apparatus can be visualized and, simultaneously, surface lubricity required in the procedures particularly in the vascular areas can be developed.    Patent Document 1: JP-T-2002-516132    Patent Document 2: Japanese Patent Laid-open No. 2005-239641    Patent Document 3: JP-T-2005-525176    Patent Document 4: JP-T-2006-503594    Non-patent Document 1: Proceedings of the 32nd Meeting of the Japanese Society for Magnetic Resonance in Medicine, 2004, p. 185, 181-22A